A number of stent-grafts for treating abdominal aortic aneurysms have been described or manufactured and many of the currently available commercial designs involve the combination of ‘Z-stents’, similar to the Gianturco (Cook Inc, Indianapolis) and a conventional tubular vascular graft woven from polyester. ‘Z-stents’ (FIG. 2) are formed from metal wire such that the path of the wire lies on the surface of a cylinder and zig-zags repeatedly between the ends of the cylinder as the wire progresses around the circumference. Usually, the two ends of the wire are joined by welding, crimping or other means to provide a single resilient structure which is of low bulk and is capable both of being compressed radially and of expanding radially once compression forces have been removed.
The characteristics of the ‘Z-stent’ can be adjusted for any given diameter by controlling the length of cylinder enclosed by the stent, the number of zig-zags made by the wire around the circumference of the cylinder and the physical characteristics of the wire. Further modifications and improvements to the basic design of the Z-stent have been employed, generally to reduce stress at the Z bends in the construction. FIGS. 3 a, b, c and d illustrates variants of bend which have been employed. Struts in Z-stents have also been modified, so that they are curved rather than straight, to permit attachments for barbs or to ease assembly of devices. The present invention applies equally to variants of Z-stents as it does to the basic structure.
Two examples of stent-grafts employing “Z-stents” are the Medtronic ‘Talent’ device and the Cook “Zenith” device. These implants employ multiple ‘Z-stents’ which are sewn at intervals along the length of a tubular woven graft in such a way as to hold the graft open and to wedge the assembly within the artery in which it is deployed. The entire assembly can be compressed radially so that it will fit into a delivery catheter, providing the means for introducing the implant into the lumen of a patient's aorta via a minimal incision into the patient's femoral or iliac artery.
The ‘Z-stent’ is not capable of being flexed along its central axis and is prone to collapse partially when it is flexed. For this reason, stent grafts comprised of ‘Z-stents’ have limited, segmental flexibility, being inflexible in the regions of the stents and partially flexible at the gaps.
An alternative reinforcing structure to the Z-stent is a tube with perforated walls so that once radially expanded, the tube has roughly diamond-shaped perforations. Such reinforcements are used in the Anneurx product from Medtronic and the Cordis stent graft. The diamond mesh structures are generally stiffer than the wire zig-zags of the Z-stent, limiting the flexibility of the overall structure in which they are used.